A novel austenite-martensite dual-phase steel with a ductility of ~30% and tensile strength over 1.4 GPa was developed. The hard martensite in the dual phase steel was strengthened through precipitation strengthening by Cu/NiAl precipitates, forming the maraging phase. The deformation mechanisms of the steel were investigated using in situ neutron diffraction and transmission electron microscopy (TEM). The results indicate that the maraging phase constrains the deformation of soft austenite, forming a strong skeleton frame with the soft austenite involved in the frame. The yield strength was controlled by the deformation of hard maraging phase, leading to the high strength of the steel. The plasticity of the maraging phase was improved through the synchronously deformation and rotation of martensite grains along with the frame-structure effect. During deformation of the maraging phase, the transfer of the dynamic stress and strain from the hard phase to a soft one compels the cooperative deformation of the soft phase together with the hard phase. This deformation contributes further to the ductility through the transformation-induced plasticity (TRIP) effects of the soft austenite. Furthermore, the cooperative deformation and the dynamic stress/strain partitions can effectively suppress the strain localization at the phase interface, retarding the crack initiation. Display omitted •A frame-structured dual phase microstructure was developed.•Strengthened martensite forms skeleton frame with the soft austenite involved.•Hard phase with frame effect controls the yielding behavior of the steel.•The cooperative deformation and stress partitions suppress the strain localization.